This invention relates generally to programmable cardiac stimulating devices. More specifically, the present invention is directed to an implantable stimulation device and associated method for automatically monitoring atrioventricular conduction time and for providing automatic adjustment of an AV hysteresis interval.
Dual-chamber pacemakers and implantable cardioverter defibrillators require manual programming of numerous programmable parameters including but not limited to: choice of pacing mode, atrioventricular delay, atrioventricular hysteresis (AV hysteresis), and other parameters such as atrial sensitivity, ventricular sensitivity, post-ventricular atrial refractory period, post-ventricular atrial blanking period, ventricular refractory period, ventricular output, atrial output, upper rate limit, base rate, sleep rate, sensor slope, sensor threshold, and so forth. The programming of these parameters can be inaccurate and time consuming, and requires skilled medical expertise to accomplish.
For example, the choice of the pacing mode is a therapeutic decision made by the medical practitioner at the time of device implant. However, the optimal pacing mode may and does change over time as the patient""s clinical condition or disease state changes. For example, a patient requiring dual chamber stimulation may in fact have intermittent atrioventricular conduction. At times when atrioventricular conduction is intact, single chamber atrial stimulation, or AAI mode, has been found to be therapeutically superior to dual chamber stimulation. Generally, such a patient would be paced in a dual chamber mode, for example DDD mode, but when an intrinsic R-wave is detected, ventricular stimulation is inhibited to allow natural heart conduction to occur. Stimulation devices capable of such functional mode switching are readily available.
A problem arises, however, in that the atrioventricular interval, which is the interval that must expire without R-wave detection following an atrial stimulation pulse before a ventricular stimulation pulse is delivered, is typically programmed to a very short value during dual chamber stimulation. A relatively short atrioventricular interval, also referred to as the AV interval has been found to give hemodynamic benefit during dual chamber stimulation in some special circumstances like hypertropic cardiomyopathy, and when AV block is present but may be deleterious in the setting of normal conduction and a normal ventricular activation sequence. A short AV interval, however, is likely to be shorter than the natural atrial-ventricular conduction time (referred to as AV conduction time) of the heart. A short AV interval will preclude the detection of the intrinsic R-waves when AV conduction is intact because ventricular stimulation will occur before natural AV conduction has had time to occur. This situation may be deleterious in the setting of normal AV conduction and a normal ventricular activation sequence. The stimulation device usurps control over the natural conduction of the heart.
One disadvantage of this pacemaker competition with natural heart conduction is that natural AV conduction, when intact, has been found to be more beneficial to the patient than dual-chamber stimulation. Another disadvantage is that predominate ventricular stimulation in a patient with intact AV conduction unnecessarily wastes pacemaker battery life.
These problems have been addressed by adding positive hysteresis to the AV interval. AV hysteresis is an additional time period added to the AV interval during ventricular sensing. In essence, AV hysteresis extends the interval that must expire before a ventricular stimulation pulse is delivered, allowing more time to sense for naturally conducted R-waves. Once ventricular stimulation is initiated, the ventricular stimulation pulses are then delivered at the programmed AV interval for a variable duration based on time or number of cycles.
The AV hysteresis is typically a programmable value that can be enabled or disabled. If enabled, it is commonly programmed to a setting between approximately 10 and 120 msec. Programming of the AV hysteresis, however, has been confusing to medical practitioners in that, first, the resulting stimulation rate is different than the sensing rate. Second, to determine an appropriate AV hysteresis, the AV interval must be temporarily programmed to a very long interval, then the AR interval (defined as the time interval between an atrial stimulation pulse and the subsequently sensed R-wave) must be measured. The minimum AV hysteresis is preferably the difference between the programmed AV interval and the measured AR interval. Since this measurement can be a time-consuming task, in practice, an arbitrary setting is often chosen. An arbitrary setting, however, may cause problems in that an insufficiently long hysteresis interval may result in fusion beats. Too short of an AV hysteresis setting would be ineffective because it does not allow a greater degree of R-wave detection.
Furthermore, a problem still exists, in that once ventricular stimulation is initiated at the programmed AV interval, it will continue to predominate over the natural heart rhythm. Attempts in overcoming this problem generally include temporarily extending the programmed AV interval by the AV hysteresis interval periodically during ventricular stimulation to allow for detection of an intrinsic R-wave in case AV conduction has returned.
While that method allows for periodic detection of restored AV conduction, the effect of temporal changes in the AV conduction time has not been fully addressed. Conventionally, the AV hysteresis is set to a fixed value that is either added to the AV interval as an additional time-out interval or not. The initial programmed value for the AV hysteresis may become inappropriate if changes in the AV conduction time occur. The AV conduction time may vary over time as a result of changes associated with the disease state of the patient, response to alterations in medical therapy, and even natural fluctuations occurring over a 24-hour period.
Thus automatically measuring the AV conduction time and monitoring variations in the AV conduction time over time would be desirable. Furthermore, automatic adjustment of the AV hysteresis based on the measured AV conduction time would be desirable to prevent pacemaker competition with natural heart conduction and thereby preserve battery life, as well as improve the performance of functional mode-switching stimulation devices in providing optimal dual-chamber stimulation therapy. As used herein, xe2x80x9cfunctional mode-switchingxe2x80x9d refers, for example, to a DDD mode that behaves like one of the other modalities such as AAI, when conduction is present.
Since the AV conduction time can be different following an atrial stimulation pulse than following an intrinsic P-wave, determination of the AV conduction time following both events would be desirable so that a unique positive hysteresis interval could be determined and applied during atrial sensing (following atrial P-waves) as well as during atrial stimulation. In addition, storing the AV conduction time measurements over time so that they are available for future display would provide a valuable a diagnostic tool for the clinician in monitoring the progression of conduction disease or responses to medical therapy.
The present invention provides an implantable cardiac stimulation device and method for periodically measuring AV conduction time and automatically adjusting one or more hysteresis intervals, for example, both AV and PV hysteresis intervals. In accordance with the present invention, AV conduction time is measured following either a stimulated or sensed atrial event by increasing the AV interval AVI (atrial-ventricular interval during atrial stimulation) or the PV interval (atrial-ventricular interval PVI during atrial sensing) by a predetermined or programmable amount (also referred to as delta) and by sensing for an intrinsic R-wave.
If an intrinsic R wave is detected within the extended AVI or PVI, the AVI or PVI remains at the longer value, respectively. If AVI were to be extended in small increments rather than the full delta, the shortest AVI or PVI at which consistent R-wave detection occurs would represent a measurement of the AV conduction time according to the stimulation device.
If the stimulation device is stimulating in the atrium when the algorithm for measuring AV conduction time is initiated, the AV interval is progressively increased in order to measure the AV conduction time between an atrial stimulation event and an intrinsic ventricular event, referred to as the AR interval (or ARI). A positive AV hysteresis (or AVH) is then calculated and can be used to automatically adjust the AV interval.
In another embodiment, the algorithm temporarily reduces the base stimulation rate in order to inhibit atrial stimulation and measure the AV conduction time between a sensed intrinsic atrial event (a xe2x80x9cP-wavexe2x80x9d) and an intrinsic ventricular event (an xe2x80x9cR-wavexe2x80x9d), referred to as the PR interval. A positive PV hysteresis is then calculated and can be used to automatically adjust the PV interval.
If the stimulation device is sensing in the atrium rather than stimulating when the algorithm is initiated, the PR interval and the PV hysteresis are determined first. Thereafter, the base stimulation rate is temporarily increased to a setting greater than the sensed atrial rate in order to induce atrial stimulation and enable measurement of the AR interval and determination of the AV hysteresis interval second.
The process or algorithm for measuring the AR interval, adjusting the AV hysteresis, measuring the PR interval, and adjusting the PV hysteresis may be enabled on a one-time basis for automatic adjustment of the hysteresis settings, or it may be repeated periodically with the AR and PR interval measurements stored in memory. In this way, the present invention provides a method for chronic monitoring of the AV conduction time by measuring changes in the AR interval and PR intervals over time.
The present invention thus achieves two main goals: 1) automatic adjustment of the positive hysteresis setting associated with AV and PV intervals so that the stimulation device appropriately responds to changes in the intrinsic AV conduction time, and 2) automatic monitoring storage and chronological reporting of the AV conduction time measurements, following both atrial sensed events and atrial stimulation, for diagnostic purposes.
The foregoing and other features of the present invention are realized by providing an implantable, cardiac stimulation device equipped with cardiac data acquisition capabilities and functional mode switching. A preferred embodiment of the stimulation device includes a control system for controlling the operation of the device; a set of leads for receiving cardiac signals and for delivering atrial and ventricular stimulation pulses; a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the cardiac signals; a sampler, such as an A/D converter for sampling cardiac signals; and pulse generators for generating atrial and ventricular stimulation pulses. In addition, the stimulation device includes memory for storing operational parameters for the control system, such as stimulation parameter settings and timing intervals like AV and PV intervals and AV and PV hysteresis intervals. The device also includes a telemetry circuit for communicating with an external programmer for transferring cardiac data, such as AV conduction times stored in the implantable device, to an external display.